Knowing how tissue interactions affect physiology is important for understanding pathologies of hormonally regulated organs. In the uterus, tissue interactions mediate steroid hormone effects, playing key roles in events of early pregnancy. Although estrogen regulates cell signaling in a stroma-to-epithelium direction in the uterus, a paracrine factor mediating this effect has not been identified. Also, hormonal regulation of a pathway in the reciprocal direction has not been studied. In both human and rodent, estrogen stimulation of a progesterone dominated uterus causes proliferation of the endometrial stroma. Proposed studies will test the hypothesis: 1) Progesterone priming for and estrogen induction of cellular proliferation in the uterine stroma occurs indirectly through activation of progesterone receptor (PR) and estrogen receptor-alpha (ER alpha) in the overlying epithelium. 2) IGF-I is a paracrine mediator of steroid action in the uterus. The hypothesis will be tested using tissue recombinations grown in xenograft. Uterine epithelium (epi) and mesenchyme (mes) will be derived from neonatal wild-type mice, ER alpha knockout (ER alpha KO) mice, PR knockout (PRKO) mice, or IGF-I knockout mice. These knockout models were chosen because: ER alpha KO mouse uterus shows no growth response to estradiol stimulation, indicating that it is devoid of any growth-promoting ER; PRKO mouse uterus exhibits no stromal response to progesterone/estrogen treatment or to a decidualizing stimulus; in the IGF-I knockout mouse uterus estrogen stimulated cells are arrested in G2. The specific aims are: 1. Determine the tissue specificity of the steroid receptor-mediated events regulating cell proliferation. 2. Determine the role of tissue specific expression of IGF-I in mediation of steroid induced proliferation in the uterus. For each type of knockout animal, tissue will be separated and recombined in all 4 possible combinations: epi+/mes+, epi-/mes-, epi+/mes-, epi-/mes+ (+, target gene status). Tissue recombinants will be grown in athymic mice. Effects of hormone treatments will be tested using tritiated thymidine incorporation or mitotic index as the endpoint. If hypothesis 1 proves correct, this will be a novel demonstration of hormonal regulation via an epithelium-to-stroma interaction, thereby leading to a new concept of hormone action in steroid-responsive organs. If IGF-I proves critical for tissue interactions it will be the first paracrine factor to be definitively identified as a mediator of steroid action in the uterus.